This invention concerns a rolling method for thin flat products, and the relative rolling line.
The invention is applied in the field of flat rolled stock, such as strip or sheet, and more particularly to obtain thin final products, around 1.5-3.0 mm, and very thin, less than 1 mm, to as little as 0.6-0.7 mm.
The state of the art includes rolling lines for flat rolled products, both of the traditional type, where the furnace to heat the slabs is separated from the casting machine and connected to the rolling train, and also the type where the rolling train is directly connected to the continuous casting machine.
The standard configuration of these rolling lines generally comprises, downstream of the continuous casting machine and the shears, a temperature-maintaining and possibly a temperature-restoring system, a roughing train comprising a number of stands which is normally between one and three, a temperature maintaining and equalisation system and the finishing train, normally comprising between four and ten stands, followed by a cooling system and a coiler to form the coil of rolled product.
Apart from these basic components of the line, normally there are also fast heating devices, for example induction furnaces, scaling devices at the outlet and/or inlet of the temperature restoring systems, devices to heat the edges, emergency shears, intermediate coilers, devices to measure the size and other operative and/or conditioning assemblies which are known to the state of the art and are widely known to those skilled in this field.
In the field of flat rolled products, obtaining strip or sheet with a final thickness of around 1.0-1.5 mm has been widely explored and the results obtained, in terms of the quality of the finished product, can be considered substantially satisfactory.
It should be considered that at the inlet to the finishing train there are usually temperature equalisation and temperature restoration systems by means of which it is possible to determine extremely rigorous and precise conditions of the product; this guarantees that at least the first passes to reduce the thickness can be carried out in the best possible rolling conditions, for example with the product in the austenitic state .gamma. and in heat conditions far removed from the allotropic transformation step from the austenitic state .gamma. to the ferritic state .alpha..
This requirement derives from the need to maintain constant values of rolling force and rolling torque in order to have a constant behaviour of the rolling stands, so that the quality of the rolled product is constant.
However, there is a tendency in the markets at present to demand ever thinner final products, less than 1.0 mm, to 0.6-0.7 mm and even less, or for products of conventional thickness, for example in the range of 1.5-3.0 mm, but with special mechanical and metallurgical characteristics.
Rolling ultra-thin products has demonstrated problems and disadvantages which had never been thought of before, which in conventional rolling lines of the type described above have a considerable impact on the surface and internal dimensional quality (thickness, profile and planarity) of the finished product.
To be more exact, it has been found that the final reducing passes of the finishing train, which obtain this kind of reduced thicknesses, are performed on a product which, in the final rolling stands, is in a condition of allotropic transformation from the austenitic state .gamma. to the ferritic state .alpha..
The temperature at which this transformation takes place depends mainly on the composition of the steel being worked and particularly on the percentage of carbon therein, the speed of cooling and the sequence of reduction of the thickness.
In the most frequent cases found in practice, that is, in steels with a low carbon content, the temperature at which the allotropic transformation begins is around 800-880.degree. C. It is highly inadvisable therefore to carry out rolling operations at such a temperature, when the product is in the process of changing from state .gamma. to state .alpha..
This is because the lack of structural homogeneity of the product, also due to the fact that it is very thin, leads to a product being obtained which is inferior in both surface and internal quality, with insufficient planarity, longitudinally uneven, with cracks and fissures, especially on the edges of the strip, and still other problems.
Until now, these problems have prevented flat rolled products being produced which are both thin and very thin and characterised by a high standard of quality.
This is also due to the need to maintain the basic structure of the rolling line, for reasons of both economics and production, so that it is possible to obtain, on the same rolling line, a relatively wide range of thicknesses as the production cycles are varied.